Molecular Beam Kinetics: Reactions of K and Cs Atoms with Unsaturated Halogenated Hydrocarbons

Crossed‐beam studies have been made of the reactions of K and Cs atoms with C₆H₅I, ${\mathrm{CH}}_2={\mathrm{CHCH}}_2\mathrm{I}\mathrm{,\hspace{0.17em}}{\mathrm{CH}}_2=\mathrm{CHI}\mathrm{,\hspace{0.17em}}{\mathrm{CH}}_2={\mathrm{CHCH}}_2\mathrm{Br}\mathrm{,\hspace{0.17em}}{\mathrm{CH}}_2=\mathrm{CHBr}$ , and ${\mathrm{CH}}_2={\mathrm{CHCH}}_2\mathrm{Cl}$ . For these systems it was found that: (1) C₆H₅I, ${\mathrm{CH}}_2={\mathrm{CHCH}}_2\mathrm{I}$ , CH₃I (included for comparison), and ${\mathrm{CH}}_2={\mathrm{CHCH}}_2\mathrm{Br}$ all gave comparable reaction cross sections ranging from ${\text{∼ 20–30 Å}}^2$ with K and ${\text{∼ 70–120 Å}}^2$ with Cs. ${\mathrm{CH}}_2=\mathrm{CHI}$ was much less reactive (${\text{≲5 Å}}^2$ ) and the remaining systems showed no measurable reaction. These results are compared with rate constants obtained from the early Polanyi sodium flame experiments. (2) The more reactive unsaturated halides produced nearly isotropic cesium halide product distributions in the c.m. system, whereas the corresponding potassium halide distributions peaked somewhat more strongly in the backward direction, although not as strongly as for CH₃I. (3) Rough estimates of the final relative kinetic energy E′ of the products obtained from matching redundant branches in the LAB→c.m. transformations indicate that a substantial portion of the chemical energy released appears as internal excitation of the products. (4) There is some evidence for the formation of a long‐lived collision complex in the reactions of C₆H₅I; the form of the product angular distribution indicates the complex is an oblate rotor in the critical configuration.